the link you refer to talks about collabora and allotropia. the first is what powers Nextcloud Office/Collabora Office online and mobile apps (using coolwsd). the latter is allotropia, who is behind Zeta that is mentioned here. both have a slightly different approach to how this works. as i understand it, all of this is upstream and not really frozen; perhaps that page needs an update?

Note: "LibreOffice Online is not intended as a standalone software. His goal is, instead, to be integrated with other tools to edit their documents." ... while Zeta and Collabora are standalone.

It wasn't frozen, Collabora were the ones doing the work and so they have largely hosted and developed LibreOffice Online.